The role of p53 as a classical tumour suppressor has been well established.1 Biochemically, p53 functions as a stress-activated sequence-specific transcription factor that activates transcription from promoters that harbour a p53 consensus-binding site.2 In addition, p53 also functions as a potent repressor of transcription, thereby adding a further layer of gene regulation.3 As such, it protects cells from a variety of stress signals such as DNA damage, nucleotide depletion, and oncogene activation to name a few, by activating the transcription of a cadre of genes involved in cell cycle arrest, apoptosis and DNA repair in addition to repressing genes involved in angiogenesis, anti-apoptosis, and cell cycle progression. The physiological consequence of p53 activation essentially leads to growth arrest or apoptosis, thereby preventing cells from replicating a genetically compromised genome.
p53 is a potent tumour suppressor protein26,27 that is negatively regulated or mutated in some, if not all, cancers. The high frequency of alternations in the p53 gene, or deregulated components of the p53 pathway, in human malignancies underscores the importance of p53 integrity to prevent carcinogenesis. This is further substantiated with the observations from the p53 knockout mice that develop spontaneous tumors by 6 months of age.4 The actual frequency of p53 gene alterations in cancers is estimated to be 20-80%. The wide variation may be attributable to the tissue of tumor origin, detection methods, and/or the regions of the gene that are analysed. For example, in breast tumours the estimated frequency of gene alteration is about 20%, whereas this frequency dramatically increases to >70% in cases of small cell lung carcinomas.5,6 Ovarian tumors generally have a wild type p53 gene28.
p53 is rapidly turned over in unstressed cells by a proteasome-dependent pathway by substrate recognition for E3 ligases such as Pirh27 and MDM2,8-10 which transfer ubiquitin from an E2 enzyme, such as UbCH5b, to a substrate on multiple lysine residues, or upon substrate-conjugated ubiquitin to generate a polyubiquitin chain. Once the K48-linked polyubiquitin chain length reaches 4 or more, the substrate can then be recognized by components of the proteasome such as hRad23a11, targeting the substrate for degradation. Thus, MDM225 and Pirh27 are negative regulators of p53. Pirh2 and MDM2 are p53-inducible genes7,12,13 thereby creating a negative feedback loop that may be employed to turn off the p53 response and allowing cell cycle progression.
Arabidopsis thaliana COP1 is a RING finger-containing protein that functions to repress plant photomorphogenesis. AtCOP1 controls seedling development by negatively regulating light-mediated gene expression14 and microarray analysis indicates that AtCOP1 regulates most, if not all, genes that are light-responsive15,16. This can be exemplified by loss-of-function mutants of the COP/DET/FUS proteins that display a phenotype that is representative of light-grown plants in darkness 7. Mechanistically, this has been attributed to AtCOP1's ability to repress positive regulators of light-mediated development such as LAF118 and HY519. COP1 has inherent E3-ligase activity in vitro29,30 and can utilise LAF1 as a substrate. While COP1 is a critical light-mediated development switch in plants, its role in mammalian cells is less well established20.